1. Field of the Invention
This invention relates to the generation of color graphic output, and more specifically to the determination of the amount of colorant used to express a color in a given area of a color graphic image.
2. Background Art
Color printers used with computer systems can reproduce thousands of shades of color by using different combinations of inks. The ink colors typically are cyan, magenta, yellow and black. A problem with current color printers is the inability of the printer paper to absorb all of the ink that could be printed on the paper. This limits the number of different colors that can be printed. This problem is understood by the following background information on color printing.
To generate a color image on a color printer, a color output device such as a computer typically transfers or applies a number of colorants (e.g., cyan, magenta, yellow and black ink or toner) to an output media (e.g., paper). The color that is reproduced on the output media is dependent on the percentage of each colorant applied to the output media. The amount of a colorant can be expressed as a percentage. For example, a color can be expressed as a percentage of cyan, magenta, yellow and black (CMYK) colorants. The total amount of colorant applied to a given region of an output medium is the combined amounts of CMYK.
It may be necessary to limit the total amount of colorant applied to an output media. The output media may not be capable of absorbing the total amount of colorant (i.e., the combined amounts of CMYK), for example. This minimizes the possibility that the colorants will bleed, run or smear, for example. Further, by reducing the amount of expensive color ink/toner, the cost of the output can be reduced. Thus, it is beneficial to limit the total amount of colorant used to output a color image.
A total area coverage (TAC) amount represents a measure of the total amount of ink (or colorant) that is to be applied to a given area of the output media. TAC is typically expressed as a percentage. For example, if 100% of each of the CMYK colorants are applied to the same area, the TAC is 400%. Similarly, if 50% of each of the four colorants are applied to the same area, the TAC is 200%. It may be necessary to specify a TAC amount, for example, to ensure that the ink can be absorbed by the output media, or to reduce the cost of generating the output.
One technique that has been used to limit the total amount of colorant is Gray Component Replacement (GCR). GCR replaces a gray component of a color with an equal amount of black color. The gray component is comprised of a mixture of C, M and Y colors. Thus, it takes a combination of three colors to produce the gray component of a color. The mixture of the C, M and Y colors used to generate the gray component produces a muddy brown color. GCR replaces the muddy brown color produced by the CMY mixture with black. Thus, only one color (i.e., black) is used to generate the gray component instead of the three C, M and Y colorants. The overall ink used to generate the color is, therefore, reduced.
Another technique used to limit the amount of ink used to generate a color is Undercolor Removal (UCR). UCR replaces equal proportions of CMY with K in the shadow tones of an image. Since the three CMY colorants are replaced with the one K colorant, the total amount of colorant applied to an area is reduced.
Both the GCR and UCR techniques produce a total amount of colorant, a GCR/UCR total, that is to be applied to a given area in which the amount of CMY is decreased and K is increased by some amount. The color produced by a CMY colorant combination has a denser or richer appearance than a color produced by the K colorant. Thus, the substitution of K for CMY in the GCR/UCR schemes can result in a duller, diluted looking image.
The GCR and UCR techniques attempt to limit the colorants used by applying a single colorant amount in place of three colorant amounts. However, even with this substitution, the GCR and UCR techniques do not ensure that the resulting GCR/UCR total has been sufficiently reduced such that the remaining colorant levels satisfy an allowable TAC amount.
Thus, current GCR/UCR techniques can overly reduce the saturation levels of color images and/or fail to ensure that a resulting GCR/UCR colorant satisfies a combined colorant level (e.g., a threshold TAC amount). For example, current GCR/UCR techniques can result in an overreduction in the colorant levels. That is, the reduction in one or more of the colorants used to generate a color as determined by a GCR/UCR technique can be greater than needed to achieve a desired goal (e.g., to ensure that the combined colorants can be absorbed by the printer paper). An overreduction caused by a GCR/UCR technique can result in a diluted image. Alternatively, current GCR/UCR techniques can cause an underreduction in the colorant levels. That is, despite the reductions performed by the current GCR/UCR techniques, the combined colorant level is greater than that allowed to ensure that the inks can be absorbed by the paper.
Background References
The following U.S. patents pertain to color image processing:
U.S. Pat. No. 4,831,409 to Tatara et al. discloses a color image processing system having a UCR function. Tatara et al. sequentially print three color component images (C, M, Y), followed by a black component image (K) so that only a single frame memory is required. Tatara et al. provide switchable operation of the UCR function.
U.S. Pat. No. 5,018,085 to Smith, Jr. teaches a system for receiving R, G, and B data in digital form, such as produced by an artist using a "paint" program on a monitor screen, and converting such data to CMYB data for printing. The system independently determines tone correction values for gray balance control and color values. The system further provides for calibration procedures.
U.S. Pat. No. 5,077,604 to Kivolowitz et al. provides a system for converting from an RGB color space to a CMYK color space and adding black ink to a resulting color such that the application of black ink is more toward colors of neutral tone and less toward saturated colors. Kivolowitz calculates a saturation value, S, for a color defined by R, G, and B values using the formula ##EQU1## A percent color removal (PCR) function of the form m(1-S.sup.1/4) is used to determine the amount to be removed from the R, G, and B color values. The R, G and B values are inverted (i.e., subtracted from 1) to convert them to C, M and Y values. The amount of black that is added back is determined by multiplying the result of the PCR function by the average of the C, M and Y values.
U.S. Pat. No. 5,087,126 to Pochieh (Hung) '126 teaches a method of estimating colors for color image correction. The method is suitable for the formation of a look-up table (LUT) in a color correcting apparatus of a video printer or a digital color copying machine. According to the method, when a specific area in which a target value T' of a given colorimetric system is present is obtained, a combination of fundamental colors T corresponding to the target value T' is calculated. The combination of fundamental colors T is calculated based on the values of the colorimetric system surrounding the target value T' and a combination of fundamental colors corresponding to the values of the colorimetric system.
U.S. Pat. No. 5,339,176 to Smilansky et al. discloses a technique for calibrating a color processing device without reference to human aesthetic judgment and a technique for transforming an element of a domain of a first color printing device to an element of a domain of a second color printing device. Smilansky et al. do not teach the use of their calibration technique with UCR or GCR processes, but only to incorporate a new digital electronic color separation scanner into an existing reproduction system using automatic calibration to achieve emulation of a UCR, GCR, or UCA (under color addition) reproduction produced by the existing system.
U.S. Pat. No. 5,355,440 to Sayanagi et al. provide a method for deciding the area ratios A.sub.y, A.sub.M, A.sub.C, and A.sub.K of Y, M, C, and K inks, which reproduce a target color X, Y, Z. The method is based upon biquadratic Neugebauer equations with four unknowns using tristimulus values of 16 types of color points obtained from actual measurement of the Y, M, C, and K inks employed in actual printing.
U.S. Pat. No. 5,363,318 to McCauley provides a system for creating channel independent linear transfer functions or calibration curves for color devices such as printers, scanners, and displays. Color saturation effects are removed by scaling or normalizing to some maximum input drive level that does not saturate any of the colors.
U.S. Pat. No. 5,381,349 to Winter et al. teach a system for calibration of color in a computer display that enables recovery from user errors. A color display screen displays a calibration color patch and a comparison color patch. The user adjusts the color of the comparison color patch until a color match is perceived. The system then determine a transfer function to convert from the value that generated the comparison color to the value assigned to the calibration color. The system can also be used to compare two transfer functions to determine if they are within a transfer function threshold value of each other.
U.S. Pat. No. 5,402,245 to Motta et al. disclose a bi-level digital color printer system with improved UCR and error diffusion processes. Motta describes a UCR technique for making a color more vivid by reducing the gray component in the color. Motta determines the color's gray component (GC). A small amount (e.g., 5% of GC) is removed from GC to yield GC'. Motta then determines what, if any, portion of GC' is removed from the color and converted to black. Motta categorizes GC' as being within either a light, medium or dark gray scale range. If GC' is within a light gray scale range, the net effect in Motta is to keep GC' in C, M and Y. If GC' is within the dark gray scale range, the net effect is to remove GC' from C, M, and Y and add it back as black. If the color is within the medium gray scale range, an exponential function is used to determine the portion of GC' that is removed from C, M and Y and added back as black.
U.S. Pat. No. 5,402,253 to Seki provides a color conversion apparatus with a variable GCR ratio. The color conversion apparatus determines a reflectance p(k') of an achromatic colorant from an amount k' of the achromatic colorant, then determines chromaticity values L*, H*, and C* according to the following equations: EQU L*=p(k').sup..about.1/2 (L*.sub.0 +16)-16 EQU H*=H*.sub.0
where L*.sub.0, H*.sub.0, and C*.sub.0 denote chromaticity EQU C*=p(k').sup..about.1/2 C*.sub.0
values of the input color signals.
U.S. Pat. No. 5,459,590 to Bleker et al. disclose a method for freely selectable substitution of the achromatic part in multi-color printing with the black ink. A two-dimensional, non-linear deepening field is formed with luminance values and a two-dimensional, non-linear desaturation field is formed with saturation values taken from the original. The substitution of the achromatic part with black ink is undertaken by an iterative search within the deepening field and within the desaturation field.
U.S. Pat. No. 5,508,827 to Po-Chieh (Hung) teaches a color separation processing method and apparatus for gradually changing a black color component from the black color component of neighbor portions of a color image. A combination of Y, M, C, and K is first calculated from a color solid which is made under the condition of Y=0, M=0, C=0, and K=max. This yields a K.sub.max value. Then, a combination of Y, M, C, and K is calculated from a color solid which is made under the condition of Y=max, M=max, C=max, and K=0. This yields a K.sub.min value. A K.sub.new value is calculated as (1-.alpha.).circle-solid.K.sub.min +.alpha..circle-solid.K.sub.max. The K.sub.new value is used to determine the combination of the three colors (Y, M, C) to be used to reproduce the target color.
U.S. Pat. No. 5,553,199 to Spaulding et al. disclose a method for calibrating a four color printer capable of printing three colors and black. The method includes the steps of forming a minimum black data structure representing a minimum black strategy; forming a maximum black data structure representing a maximum black strategy; for each printable black level, forming a fixed black data structure. The method also includes the step, for a specific output color value, of finding the minimum and maximum black levels using the minimum and maximum black data structures. The method further includes the steps of determining a desired black level between the minimum and maximum black levels according to a defined black strategy, and determining the three printing color levels from the fixed black data structure corresponding to the desired black level.
U.S. Pat. No. 5,572,632 to Laumeyer et al. provide a method for image data processing using a transformation to device independent, intermediate color space color coordinates and storing such coordinates in an intermediate frame buffer. The method involves converting input color coordinates to L*, a*, and b* color coordinates for each output image pixel, storing the L*, a*, and b* color coordinates in a frame buffer, converting the frame buffer data from L*, a*, and b* color coordinates to C, M, Y, and K color coordinates. The method further involves converting the C, M, Y, and K color coordinates to dot gain corrected color coordinate data and storing the dot gain corrected data in a printer C, M, Y, and K frame buffer.
U.S. Pat. No. 5,615,312 to Kohler discloses a business graphics rendering mode that provides increased color contrast for business graphics applications, while leaving essentially unaltered achromatic neutrally-colored areas of the image. By not altering achromatic neutrally-colored areas, Kohler avoids undesirably converting light gray areas to white and dark gray areas to black.